Various aluminium alloy forgings for use as automotive suspension parts such as upper arms and lower arms have been conventionally developed.
For example, patent document 1 describes an automotive suspension part consisting of an aluminium alloy forging comprising: Mg: 0.5 to 1.25% by mass, Si: 0.4 to 1.4% by mass, Cu: 0.01 to 0.7% by mass, Fe: 0.05 to 0.4% by mass, Mn: 0.001 to 1.0% by mass, Cr: 0.01 to 0.35% by mass, Ti: 0.005 to 0.1% by mass, Zr being restricted to less than 0.15% by mass, the remainder being Al and inevitable impurities, wherein in the structure in a cross section in the widthwise direction in a portion where the maximum stress is generated, the density of crystallized grains observed in the structure in the cross section portion where the maximum stress is generated is 1.5% or lower at an average area ratio, and the interval between grain boundary precipitates observed in the structure in the cross section portion including the parting line occurring during forging is 0.7 μm or more at an average interval.
In addition, this patent document 1 describes that the size of dispersed particles observed in the structured in a cross section portion where the maximum stress is generated is 1200 Å or lower at an average diameter, and the density of these dispersed particles is 4% or higher at an average area ratio, the percentage area occupied by recrystallized grains observed in cross sectional structures of these ribs is 10% or lower at an average area ratio, and further the percentage area occupied by recrystallized grains observed in cross-sectional structures in the width direction of webs adjacent to cross-sectional structures of these ribs is 20% or lower at an average area ratio.
In addition, this patent document 1 describes a method for manufacturing an automotive suspension part comprising casting an aluminium alloy molten metal having the above-mentioned predetermined composition at an average cooling rate of 100° C./s. or higher, heating this cast ingot to a temperature range from 460 to 570° C. at a programming rate of 10 to 1500° C./hr., performing a homogenizing heat treatment in which the ingot is retained within this temperature range for 2 hours or longer and then cooling the ingot to room temperature at a cooling rate of 40° C./hr. or higher, and further re-heating the ingot to hot-forging initiation temperature to perform hot-mold forging, the forging end temperature being 350° C. or higher, performing a solution treatment for retaining the ingot for 20 minutes to 8 hours in a temperature range from 530 to 570° C. after this hot-forging, then performing a hardening process at an average cooling rate ranging from 200 to 300° C./s., and further performing artificial age hardening.
According to the invention described in this patent document 1, a forged automotive suspension part having a light-weight configuration achieving high strength, tenacity and corrosion resistance can be realized.